An Integrated Services Digital Network (ISDN) provides both voice and data telecommunications in accordance with a set of standard interfaces. Familiarization with ISDN standards and recommendations is useful to an understanding of the present invention. The Committee Consulative International Telephone and Telecommunications (CCITT) has prepared a number of Recommendations and reference should be had to the I-Series Recommendations for a complete discussion of ISDN nomenclature and standards.
The CCITT I-Series Recommendations specify a subscriber "S" interface to the telecommunications ISDN. Terminal Equipment (TE) at a subscriber's premises connects to the ISDN at the "S" interface reference point. Signals received over the "S" interface have a frame structure specified by the I-Series Recommendations. A pseudo-ternary code is employed and each frame starts with a framing (F) bit and DC balancing (L) bit. These bits are followed by data bits which are either a ONE at a zero level "space" or a ZERO which is alternately at a positive level and a negative level, a "mark". Thus, a three-level coding scheme is employed at the "S" interface.
ISDN receivers take the signal waveform at the "S" reference point and amplify it sufficiently so that the TE can adequately process the received signals. A step-up transformer is typically employed within the receiver at the "S" interface to connect the TE to the lines transferring signals over the ISDN.
Difficulty is experienced in receiving the first data bit following a framing bit and balance bit in the B1 channel and the first data bit following the (auxiliary) framing bit and balance bit in the B2 channel. The problem is especially acute when the value of the first data bit is ONE, i.e., a "space". This is because the terminal equipment on the network generating the signals (the "driver") turns off into a high impedance state and the change from the "mark" condition of the balance bit to the "space" of this first data bit is a relatively slow process. Exacerbating this situation is the fact that in short Passive Bus and Extended Passive Bus Configurations there are several TEs contributing to the framing bit, but only one TE driving the line during the data fields. Thus as TE's are spread out on the bus, there are timing delays between the various TE's. This staggering of TE causes broadening of the pulse shape received by the NT. In particular, the closest NT receives framing bits relatively earlier, and accordingly, an early data signal from the nearest TE is partially destroyed by framing bits received from TE's further removed.
Errors in data received by NT on the ISDN are introduced by the slow transition from the balance bit (a mark) to the first data bit (a space), because the space bit may be misinterpreted as a mark.